Process and system for capture of carbon dioxide

ABSTRACT

A process for absorbing carbon dioxide from a gas stream containing carbon dioxide, including contacting the gas stream with an aqueous composition having a substituted heteroaromatic compound including a six-membered heteroaromatic ring comprising from 1 to 3 nitrogen atoms in the heteroaromatic ring and at least one substituent wherein at least one of the substituents is of formula —R1NH2 wherein R1 is selected from C1 to C6 alkylene and ethers of formula —R2—O—R3— wherein R2 and R3 are C1 to C3 alkylene.

FIELD

The invention relates to a process and system for capture of carbondioxide from gas streams.

BACKGROUND

Emission of carbon dioxide is considered the main cause of thegreenhouse effect and global warming. In the Kyoto Protocol the UnitedNations Framework Convention on Climate Change has set targets for thereduction of greenhouse gas emissions.

One method of reducing atmospheric CO₂ emissions is through its captureand subsequent geological storage. In post combustion capture, the CO₂in flue gas is first separated from nitrogen and residual oxygen using asuitable solvent in an absorber. The CO₂ is then removed from thesolvent in a process called stripping (or regeneration), thus allowingthe solvent to be reused. The stripped CO₂ is then liquefied bycompression and cooling, with appropriate drying steps to preventhydrate formation. Post combustion capture in this form is applicable toa variety of CO₂ sources including power stations, steel plants, cementkilns, calciners, biogas plants, natural gas processing, methanereforming and smelters.

Aqueous amine solutions and alkanolamine solutions in particular, havebeen investigated as solvents in post combustion CO₂ capture. Thecapture process involves a series of chemical reactions that take placebetween water, the amine and carbon dioxide. Amines are weak bases, andmay undergo acid-base reactions. Once dissolved into the amine solution,the aqueous CO₂ reacts with water and the neutral form of the aminereact to generate carbamate, protonated amine, carbonic acid (H₂CO₃),aqueous bicarbonate (HCO₃ ⁻) ions and aqueous carbonate (CO₃ ²⁻) ions.

CO₂ desorption is achieved by heating of an aqueous amine solutioncontaining CO₂. The two major effects of heating are to reduce thephysical solubility of CO₂ in the solution, and to reduce the pKa of theamine resulting in a concomitant reduction in pH and in CO₂ absorptioncapacity, the net effect of which is CO₂ release. The extent of thereduction in pKa is governed by the enthalpy of the amine protonationreaction which in turn is governed by the amine chemical structure. Allthe other reactions, including carbamate formation, have small reactionenthalpies and are insensitive to temperature. Typically, the enthalpyof amine protonation is four to eight times larger than the enthalpiesof the carbonate reactions and two to four times larger than theenthalpy of carbamate formation. It is the lowering of the pH uponheating that drives the reversal of carbamate and carbonate/bicarbonateformation during desorption, rather than any significant reduction instability.

The cyclic capacity (α_(cyclic)) of an aqueous amine solution is definedas the moles of CO₂ that can be absorbed and released per mole of amineby cycling the absorbent between low temperature (α_(rich)) and hightemperature (α_(lean)): α_(cyclic)=α_(rich)−α_(lean). In terms ofchemistry, this cyclic capacity is primarily governed by the change inamine pKa with temperature. The larger this cyclic capacity, the moreefficient the amine. 30 wt % monoethanolamine, which is currentlyemployed in industrial CO₂ capture, possesses an undesirable cycliccapacity of approximately α_(cyclic)=0.11 (40° C.-80° C.).

US Publication 2015/0367281 describes a process for absorption of anacid gas which involves contacting the acid gas with a benzylaminecompound and a cosolvent which reduces the vapour pressure of thebenzylamine. The use of certain cosolvents also ameliorates the problemprecipitate formation due to low solubility of the anion formed fromreaction of the benzylamine compound with carbon dioxide.

There remains a need to identify amines and systems which provideimproved properties and/or reduce problems in carbon dioxide capture andrelease.

SUMMARY OF INVENTION

The invention provides a process for absorbing carbon dioxide from a gasstream containing carbon dioxide, comprising contacting the gas streamwith an aqueous absorbent composition comprising a substitutedheteroaromatic compound comprising an six-membered heteroaromatic ringcomprising from 1 to 3 nitrogen atoms in the heteroaromatic ring and atleast one substituent wherein at least one substituent is of formula—R¹NH₂ wherein R¹ is selected from the group consisting of C₁ to C₆alkylene and ethers of formula —R²—O—R³— wherein R² and R³ are C₁ to C₃alkylene.

In one embodiment there is further provided a composition of adsorbedcarbon dioxide comprising:

-   -   A. an aqueous solvent;    -   B. at least one absorbent compound for carbon dioxide comprising        a substituted heteroaromatic compound comprising a six membered        heteroaromatic ring comprising from 1 to 3 nitrogen atoms in the        heteroaromatic ring and at least one substituent, wherein at        least one of the substituents is, of formula —R¹NH₂ wherein R¹        is selected from the group consisting of C₁ to C₆ alkylene and        ethers of formula —R²—O—R³— wherein R² and R³ are C₁ to C₃        alkylene; and    -   C. absorbed carbon dioxide, wherein the absorbed carbon dioxide        is at a concentration above the equilibrium concentration when        the solution is exposed to air at below the boiling point of the        solvent.

In yet a further embodiment there is provided a use of an aqueoussolution of a substituted heteroaromatic compound in capture of carbondioxide in a gas stream comprising carbon dioxide, wherein thesubstituted heteroaromatic compound comprises a heteroaromaticsix-membered ring comprising from 1 to 3 nitrogen atoms in theheteroaromatic ring and at least one substituent wherein at least of thesubstituents is of formula —R¹NH₂ wherein R¹ is selected from the groupconsisting of C₁ to C₆ alkylene, C₁ to C₆ oxy-alkylene and ethers offormula —R²—O—R³— wherein R² and R³ are C₁ to C₃ alkylene.

In one set of embodiments the concentration of substitutedheteroaromatic compound is 1 wt % to 80 wt % of the aqueous composition,preferably 10 wt % to 80 wt % of the aqueous composition.

In one set of embodiments the amount of water in the aqueous compositionat least 10 wt %, preferably at least 20 wt %.

The process of the invention may also be carried out using a furtherabsorbent for carbon dioxide in addition to the substitutedheteroaromatic compound. The weight ratio of the substitutedheteroaromatic compound to further absorbent may, for example, be from1:10 to 10:1.

The substituted heteroaromatic compound of the composition provides ahigh cyclic capacity for carbon dioxide allowing efficient capture anddesorption of carbon dioxide while also providing a low vapour pressurein aqueous solution and good solubility of the substitutedheteroaromatic compound and species formed on absorption of carbondioxide. The aqueous composition may therefore be used without arequirement for specific co-solvents and provides flexibiity informulating aqueous compositions for carbon dioxide capture and release.

We have found that the long term stability of the substitutedheteroaromatic compound during CO₂ capture, particularly where R¹ ismethylene, is enhanced in the presence of a further amine of higherbasicity than the substituted heteroaromatic compound. In a preferredembodiment the absorbent further comprises an amine selected fromtertiary amines and primary and secondary sterically hindered amines andmixtures thereof having a higher basicity than the substitutedheteroaromatic compound. In particular, in the case where R¹ ismethylene such as for (aminomethyl)pyridines the further amine selectedfrom tertiary amines and primary and secondary sterically hinderedamines and mixtures thereof typically has a pKa of at least 8.85 at 25°C. A pKa of 8.85 is about 0.25 above the pKa of (aminomethyl)pyridinesspecifically each of 2-(aminomethyl)pyridine, 3-(aminomethyl)pyridineand 4-(aminomethyl)pyridine.

As used herein, except where the context requires otherwise, the term“comprise” and variations of the term, such as “comprising”, “comprises”and “comprised”, are not intended to exclude further additives,components, integers or steps.

Detailed Description BRIEF DESCRIPTION OF DRAWINGS

Examples of the invention are described with reference to the attacheddrawings.

In the drawings:

FIG. 1 is a schematic diagram of a system used for removing carbondioxide from a gas mixture according to the present invention.

FIG. 2 show part sectioned views of Wetted Wall Column (WWC) apparatusand the column portion of the WWC apparatus used in evaluating andcomparing CO₂ absorption rates.

FIG. 3 is a graph including 5 plots showing the overall CO₂ masstransfer coefficients, K_(G), at 40.0° C. for a series of aqueous(aminomethyl)pyridine (AMPy) absorbents and aqueous monoethanolamine(MEA) from 0.0-0.4 CO₂/mole amine. The (aminomethyl)pyridines examinedinclude 2-(aminomethyl)pyridine (2-AMPy), 3-(aminomethyl)pyridine(3-AMPy) and 4-(aminomethyl)pyridine (4-AMPy) each at a concentration of6.0M. Monoethanolamine (MEA) is included for comparison. The plots, atleft hand side of the graph are in the order of, from to bottom, 2-AMPy,3-AMPy, 4-AMPy, MEA.

FIG. 4 is a graph of vapour liquid equilibria data comparing the amountof CO₂ absorbed as a function of CO₂ partial pressure for 6.0M 3-AMPyand 2.6M BZA each at 295K and 313K and 333K. The additional capacity ofthe more concentrated 3-AMPy absorbent is apparent.

FIG. 5 is a graph having 5 plots showing the overall CO₂ mass transfercoefficients (K_(G)) at 40° C. for a series of 4-AMPy absorbents andblends of from 0.0 to 0.4 moles CO₂/moles amine. The plots include, fromtop to bottom at the left hand side of the graph: 3M 4-AMPy+3.0M MEA,6.0M 4-AMPy, 6.0M MEA, 3.0M 4-AMPy+3.0M DMEA and 3.0M 4-AMPy+3.0M AMP.

FIG. 6 is a graph including 4 plots viscosity of absorbent solutions at40° C. against CO₂ loadings from 0.0-0.4 moles CO2/total moles amine foraqueous absorbent solution including absorbent solutions of, from top tobottom at the left hand side of the graph: 3.0M 4-AMPy+3.0M AMP; 3.0M4-AMP+3.0M DMEA; 6M 4-AMPy; and 3.0M 4-AMPy+3.0M MEA.

FIG. 7 is a graph having 5 plots comparing the model estimated reboilerduty (kJ/kg CO₂) for a range of absorbents as a function of the ratio ofliquid and gas flow rates (L/G) (kg/kg) for absorbents from top tobottom at right hand side of the graph of 30 wt % MEA; 4:2 3-AMPy:AMP;5:1M 3-AMPy:AMP; 6M 3-AMPy; 3:3M3-AMPy:AMP.

FIG. 8 is a graph comparing the reboiler energy requirement (GJ/tonneCO₂) with increasing liquid/gas flow ratio (kg/kg) for 90% capture at101.3 kPa (15 kPa CO₂) for aqueous 6M 3-AMPy against 5M (30 wt %) MEA.

FIG. 9 is a graph showing the 3-AMPy and imine dimer concentration by IRspectroscopy (solid points) and HPLC (circles) over 1500 hours ofoperation.

FIG. 10 is a graph showing the change in concentration with time ofoperation of the pilot plant of 3-AMPy, AMP and imine dimer formed from3-AMPy using the aqueous absorbent solution containing 3M 3-AMPy and 3MAMP in capture of CO₂.

FIG. 11 is a graph which compares the enthalpy and entropy ofprotonation of a range of amines. The red squares allow comparison of 4different aminomethyl aromatic compounds and 2-phenyethylamine toillustrate the difference between a methyl linking group and a linkinggroup of an alkyl chain specifically ethyl.

The aqueous absorbent composition for absorption of carbon dioxide fromthe gas stream comprises an optionally substituted six-memberedheteroaromatic ring comprising from 1 to 3 nitrogen atoms in theheteroaromatic ring and at least one substituent wherein at least onesubstituent is of formula —R¹NH₂ wherein R¹ is selected from the groupconsisting of C₁ to C₆ alkylene and ethers of formula —R²—O—R³— whereinR² and R³ are C₁ to C₃ alkylene.

In one set of embodiments the optionally substituted six memberedhetero-aromatic ring is of formula (I):

wherein

X is independently selected from N and the group CR where at least threeof X are CR;

R are independently selected from the group consisting of hydrogen, C₁to C₄ alkyl, hydroxy, hydroxy-C₁ to C₄ alkyl, C₁ to C₄ alkoxy, C₁ to C₄alkoxy-(C₁ to C₄ alkyl) and at least one R is the group of formula—R¹—NH₂ wherein R¹ is selected from the group consisting of C₁ to C₆alkylene, C₁ to C₆ oxyalkylene and ethers of formula —R²—O—R³— where R²and R³ are C₁ to Ca alkylene. R¹ is most preferably methylene.

Preferred compounds of formula I are selected from formula Ia, Ib, Ic,Id and mixtures of two or more thereof:

wherein

R¹ is a carbon substituent and is selected from the group consisting ofC₁ to C₆ alkylene and ethers of formula —R²—O—R³ wherein R² and R³ areC₁ to C₃ alkylene and R¹ is most preferably methylene;

R⁴ is an optional carbon substituent selected from the group consistingof C₁ to C₄ alkyl, hydroxy, hydroxy —C₁ to C₄ alkyl, C₁ to C₄ alkoxy. C₁to C₄ alkoxy-(C₁ to C₄ alkyl); and

n is 0, 1 or 2.

Preferably formula I is of formula Ia:

wherein

R¹ is a carbon substituent and is selected from the group consisting ofC₁ to C₆ alkylene and ethers of formula —R²—O—R³ wherein R² and R³ areC₁ to C₃ alkylene and R¹ is preferably methylene;

R⁴ is an optional carbon substituent selected from the group consistingof C₁ to C₄ alkyl, hydroxy, hydroxy —C₁ to C₄ alkyl, —C₁ to C₄ alkoxy,C₁ to C₄ alkoxy-(C₁ to C₄ alkyl); and

n is 0, 1 or 2.

Still more preferably the substituted heteroaromatic compound isselected from the group consisting of formula IIa, IIb, IIc and mixturesof two or more thereof:

wherein R¹ is selected from the group consisting of C₁ to C₄ alkylene,preferably methylene.

The substituted heteroaromatic compound includes a substituent group R¹which is a group that links the amino group (NH₂) to the heteroaromaticring. Specific examples of the substituent R¹ may be selected from thegroup consisting of:

—CH₂—, —CH₂CH—, —CH₂CH₂(CH₃)—, —CH₂(CH₃)CH₂—

—CH₂(CH₂—CH₃)—; and

—CH₂CH₂—OCH₂CH₂—.

Examples of optionally substituted heteroaromatic compounds are selectedfrom the group consisting of 2-[amino(C₁ to C₄ alkyl)]pyridine,3-[amino(C₁ to C₄ alkyl)]pyridine and 4-[amino(C₁ to C₄ alkyl)]pyridine.

Examples of substituted heteroaromatic compounds includeaminomethylpyridines including 2-(aminomethyl)pyridine,3-(aminomethyl)pyridine and 4-(aminomethyl)pyridine 3-aminoethylpyridine, 3-(amino-2-methylethyl)pyridine, 3-(1-aminopropyl)pyridine,3-(2-aminopropyl)pyridine or mixture of two or more thereof. Mostpreferably the substituted heteroaromatic compound is selected from thegroup of 2-(aminomethyl)pyridine, 3-(aminomethyl)pyridine,4-(aminomethyl)pyridine and mixture of two or more thereof.

The absorbent composition need not include ionic liquids or organicsalts such as imidazolium cation or quaternary ammonium salts and willtypically include a water content of more than 15 wt % of the absorbentcomposition such as at least 20 wt %.

Compositions of the substituted heteroaromatic compound, particularlythe (aminomethyl)pyridines, have lower susceptibility to thermal andoxidative degradation than a 30 wt % MEA solution due to the inherentchemical stability imparted by the heteroaromatic ring structure.Preferably the cyclic absorption capacity of the solution for CO₂ iscomparable to that of a tertiary or sterically hindered amine solutionand the rate of absorption of the target gas is comparable to or betterthan a 30 wt % MEA solution.

At least one of the substituted heteroaromatic compounds may constitutethe total of the carbon dioxide absorbent compound or may be present insolution with other suitable carbon dioxide absorbent compounds so thatthe total gas absorbent compounds comprise one or more gas absorbentcompounds in addition to the substituted heteroaromatic compound. Thesubstituted heteroaromatic compound preferably comprises at least 1 wt%, more preferably 1 wt % to 80 wt %, still more preferably 1.5 wt % to80 wt %, even more preferably 5 wt % to 80 wt %, more preferably 10 wt %to 80 wt %, such as 15 wt % to 80 wt %, 20 wt % to 80 wt % or 25 wt % to80 wt % relative to the total weight of the solution. In someembodiments the concentration of the substituted heteroaromatic compoundin the aqueous composition is from 30 wt % to 80 wt % such as 40 wt % to80 wt %, 50 Wt % to 80 wt % or 60 wt % to 80 wt %. The high solubilityof the substituted heteroaromatic compound allows high loadings to beused in aqueous solution and also provides a solution stableintermediate in the carbon dioxide absorption process. It thereforeprovides significant practical advantages in this respect when comparedwith benzyl amine and its derivatives.

The total wt % of the at least one absorbent compound (including thesubstituted heteroaromatic compound) in solution is preferably at least20 wt %, more preferably at least 25 wt %, still more preferably atleast 30 wt %, even more preferably at least 40 wt % and yet even morepreferably at least 50 wt % relative to the total weight of thesolution. This component will typically consists of the substitutedheteroaromatic compound and optionally one or more compounds selectedfrom amines and in the preferred embodiment at least one of theaminomethyl substituted heteroaromatic compounds and one or more aminesselected from tertiary amine or sterically hindered primary or secondaryamine which has a higher basicity than the substituted heteroaromaticcompounds.

In one set of embodiments the absorbent composition may comprise:

the heteroaromatic compound in an amount of 10 wt % to 80 wt % such as15 wt % to 80 wt %, 20 wt % to 80 wt % or 25 wt % to 80 wt % relative tothe total weight of the solution;

optionally a further amine absorbent for carbon dioxide in an amount ofup to 70 wt % such as from 10 wt % to 70 wt %;

water in an amount of at least 10 wt % such as from 10 wt % to 90 wt %or 20 wt % to 80 wt %.

Further components may be present such as solvents solutes or othermaterials.

When the additional amine absorbent for carbon dioxide is present theweight ratio of the substituted heteroaromatic to further amine may, forexample, be in the range of from 99:1 to 1:10 such as 1:10 to 10:1 or1:5 to 5:1.

In one embodiment the solution contacted with the gas stream comprisesone or more additional carbon dioxide gas absorbing compounds selectedfrom amines and imidazoles in addition to the substituted heteroaromaticcompound. The one or more additional amines may be selected fromprimary, secondary and tertiary amines.

Examples of suitable amines include primary amines such asmonoethanolamine, ethylenediamine, 2-amino-2-methyl-1-propanol.2-amino-2-methyl-ethanolamine and benzylamine; secondary amines such asN-methylethanolamine, piperazine, piperidine and substituted piperidine,3-piperidinemethanol, 3-piperidineethanol, 2-piperidinemethanol,2-piperidineethanol, diethanolamine, diglycolamine anddiisopropanolamine; and tertiary amines such as N-methyldiethanolamine,N-piperidinemethanol, N-piperidineethanol, N,N-dimethylaminoethanol and3-quinuclidinol; imidazole and N-functionalised imidazole and aminoacids such as taurine, sarcosine and alanine.

The process is particularly effective in capture of CO₂ in the presenceof a further amine component selected from tertiary amines andsterically hindered primary and secondary amines.

We have found through pilot plant testing that(aminomethyl)heteroaromatics, particularly the (aminomethyl)pyridines,degrade slowly when the gas stream contains a significant oxygencomponent which is often the case in combustion gas streams. Theresulting product of oxidation is an imine which is a dimer. Theformation of the dimer is understood to occur according to the schemeshown below:

The mode of degradation is considered to be unique to aminomethylsubstituted aromatics and aminomethyl substituted heteroaromatics as thesame dimer stability arising through conjugation with the aromatic ringis not possible for alkyl ckains. The above scheme and stabilisation ofthe imine is consistent with a synthetic scheme reported for thepreparation of more complex heterocyclics such as aromatic andheteroaromatic substituted benzimidazoles in Green Chem., 2013 15,2713-2717. The resulting imine is favoured as it forms a stableconjugated pi-bonding arrangement. This stabilisation does not extend toaminoalkylsubstituted heteroaromatics in which the alkyl linking groupis ethyl or longer chain alkyl which undergo degradation via similarpathways to aliphatic amines such as the formation of carboxylic adds,aldehydes and amino acids (C Gouedard, D Picq, F Launay, P-L Carrette;Int. J. Greenh. Gas Con., 10,244 (2012)).

We have found that formation of the imine and resulting degradation ofthe aminomethyl-substituted heteroaromatic is inhibited in the presenceof an amine, particularly a tertiary amine or sterically hinderedprimary or secondary amine which has a higher basicity than theaminomethyl-substituted heteroaromatic. For example a pKa at least 0.25higher than the pKa of the aminomethyiheteroaromatic. In the case of(aminomethyl)pyridines the pK_(a) of 2-(aminomethyl)pyridine,3-(aminomethyl)pyridine and 4-(aminomethyl)pyridine at 25° C. is 8.6. Itis preferred that the further amine particularly a sterically hinderedamine or tertiary amine have a pKa of at least 8.85 such as a pKa of atleast 9, a pKa of 8.85 to 11.5 or pKa of 9 to 11.5. It is consideredthat the higher basisity adsorbs protons from the acid gas in solutionthereby inhibiting the initial step in the degradation process. Inpractice the combined use of an (aminomethyl)pyridine with an amineselected from the group consisting of tertiary amines and stericallyhindered primary and secondary amines and mixtures thereof of higherbasicity, such as 2-amino-2-methyl-1-propanol bas been found to verydramatically reduce the imine formation and resultant degradation of the(aminomethyl)pyridine.

As used herein the term “sterically hindered amine” is defined as thosecompounds containing at least one primary or secondary amino groupattached to either a secondary or tertiary carbon atom. In oneembodiment the sterically hindered amine is a secondary amino groupattached to either a secondary or tertiary carbon atom or a primaryamino group attached to a tertiary carbon atom. Examples of suitablesterically hindered amines and tertiary amines include those shown inthe following table with the corresponding pKa at 25° C.

Base pK_(a) at 25° C. 2-amino-1-propanol 9.5 2-amino-2-methyl-1-propanol9.7 piperidine 11.1 2-piperidinylmethanol 10.1 3-piperidinylmethanol10.4 4-piperidinylmethanol 10.6 2-piperidinylethanol 10.54-piperidinylethanol 10.6 2-(dimethylamino)ethanol 9.2

Other suitable tertiary amines and sterically hindered amines of therequired basicity will be readily apparent to those skilled in the arthaving regard to the above reference degradation mechanism and method ofinhibition of imine formation.

Accordingly in a preferred aspect the further amine optionally presentin the composition is present in an amount of 10 wt % to 70 wt % theabsorbent. In a further aspect the absorbent comprises:

10 wt % to 80 wt % of (aminomethyl)pyridine comprising one or more of2-(aminomethyl)pyridine, 3-(aminomethyl)pyridine and4-(aminomethyl)pyridine;

10 wt % to 70 wt % of amine selected from tertiary amines and stericallyhindered primary and secondary amines and mixtures thereof of pKa atleast 8.85, preferably from 2-amino-1-propanol,2-amino-2-methyl-1-propanol, piperidine, 2-piperidinylmethanol,3-piperidinylmethanol, 4-piperidinylmethanol, 2-piperidinylethanol,4-piperidinylethanol and 2-(dimethylamino)ethanol, and most preferably2-amino-2-methyl-1-propanol; and

20 wt % to 80 wt % water.

In a further embodiment the absorbent comprises:

20 wt % to 60 wt % of (aminomethyl)pyridine comprising one or more of2-(aminomethyl)pyridine, 3-(aminomethyl)pyridine and4-(aminomethyl)pyridine;

10 wt % to 50 wt % of amine selected from tertiary amines and stericallyhindered primary and secondary amines and mixtures thereof of pKa atleast 8.85, preferably from 2-amino-1-propanol,2-amino-2-methyl-1-propanol, piperidine, 2-piperidinylmethanol,3-piperidinylmethanol, 4-piperidinylmethanol, 2-piperidinylethanol,4-piperidinylethanol and 2-(dimethylamino)ethanol, and most preferably2-amino-2-methyl-1-propanol; and

20 wt % to 70 wt % water.

The tertiary and sterically hindered amines are typically soluble in thecomposition at the desired concentration at 25° C. Preferably thetertiary and sterically hindered amines are water soluble at 25° C.

In a further embodiment, the solution comprises an amine absorbentselected from imidazole and more preferably an N-functionalisedimidazole. Suitable N-functionalised imidazoles may be found in U.S.Pat. No. 8,741,246, which is incorporated herein by reference.

The suitable N-functionalised imidazoles disclosed in U.S. Pat. No.8,741,246 are of formula (2):

wherein

R¹ is substituted or unsubstituted C₁₋₂₀ alkyl, substituted orunsubstituted C₂₋₂₀ alkenyl, substituted or unsubstituted C₂₋₂₀ alkynyl,substituted or unsubstituted C₁₋₂₀ heteroalkyl, substituted orunsubstituted C₂₋₂₀ heteroalkenyl, substituted or unsubstituted C₂₋₂₀heteroalkynyl, substituted or unsubstituted cycloalkyl, substituted orunsubstituted heterocycloalkyl, substituted or unsubstituted aryl,substituted or unsubstituted heteroaryl, substituted or unsubstitutedthio, substituted or unsubstituted amino, substituted or unsubstitutedalkoxyl, substituted or unsubstituted aryloxyl, silyl, siloxyl, cyano,or nitro; and

R², R³, and R⁴ are each independently selected from hydrogen, halogen,hydroxyl, substituted or unsubstituted C₁₋₂₀ alkyl, substituted orunsubstituted C₂₋₂₀ alkenyl, substituted or unsubstituted C₂₋₂₀ alkynyl,substituted or unsubstituted C₁₋₂₀ heteroalkyl, substituted orunsubstituted C₂₋₂₀ heteroalkenyl, substituted or unsubstituted C₂₋₂₀heteroalkynyl, substituted or unsubstituted cycloalkyl, substituted orunsubstituted heterocycloalkyl, substituted or unsubstituted aryl,substituted or unsubstituted heteroaryl, substituted or unsubstitutedthio, substituted or unsubstituted alkoxyl, aryloxyl, substituted orunsubstituted amino, cyano, or nitro.

Specific examples of such compounds include the 1-N—(C₁ to C₂₀ alkyl)imidazoles such as 1-butyl imidazole.

In some, embodiment the solution comprises a combination ofN-functionalised imidazoles and one or more amines in addition to thesubstituted heteroaromatic compound.

The one or more amines which may be used in addition to theN-functionalised imidazoles may be selected from the group consisting ofprimary, secondary and tertiary amines including the specific examplesof such amines referred to above.

In another aspect of the invention there is provided a process forremoving carbon dioxide gas from a gas mixture including: contacting agas mixture that is rich in carbon dioxide with an absorbent solution,as described above, to form a target gas rich solution and a gas mixturethat is lean in target gas; and desorbing the carbon dioxide gas fromthe solution. In yet a further set of embodiments there is furtherprovided use of a substituted heteroaromatic compound in aqueoussolution at a concentration of at least 20% by weight, based on thetotal weight of the solution, for absorbing carbon dioxide from a gasstream. Desorbing the carbon dioxide may be facilitated by an increasein temperature, reduction in pressure, change in pH or combination ofthese factors. It is a significant advantage of the invention that thecyclic capacity of COs brought about by these changes is generallyspeaking higher for the substituted heteroaromatic compound used in theprocess of the invention than the molar equivalent of other absorbentssuch as monoethanolamine.

In one set of embodiments there is further provided a compositioncomprising an aqueous solution for carbon dioxide gas comprising:

-   -   A. an aqueous solvent;    -   B. at least one absorbent compound comprising the substituted        heteroaromatic compound; and    -   C. an absorbed carbon dioxide, at a concentration above the        equilibrium concentration when the solution is exposed to air at        below the boiling point of the solvent.

Preferably, the concentration of the absorbed carbon dioxide is at leasttwo times (and even more preferably at least five times) the equilibriumconcentration when the solution is exposed to air at below the boilingpoint of the aqueous solvent, thus representing the absorbed carbondioxide concentration in the solvent during the absorption process aspreviously described. The background amount of CO₂, will generally beless than 0.1% by weight based on the total weight of the solution. Inone embodiment the absorbed carbon dioxide will constitute at least 0.2%by weight based on the total weight of the solution on absorption of thegas more preferably at least 1% and still more preferably at least 10%absorbed carbon dioxide by weight based on the total weight of thesolution.

In one embodiment the solution comprises one or more amines in additionto the substituted heteroaromatic compound which additional amines may,for example, be selected from primary, secondary and tertiary aminesoptionally including N-functionalised imidazoles such as those offormula (2).

In a set of embodiments the total of the absorbent component and waterconstitute at least 40%, preferably at least 50%, more preferably atleast 60%, still more preferably at least 70% and even more preferablyat least 80% (such as at least 90%) by weight of the total composition.

FIG. 1 provides an illustration of an embodiment of a process forcapture of carbon dioxide from a flue gas stream. The process (100)includes an absorption reactor (102), for absorbing CO₂ from a flue gasstream, and a desorption reactor (104) for desorbing CO₂.

The absorption reactor (102) includes a first inlet (106), a secondinlet (108), a first outlet (110), and a second outlet (112), and a gasabsorption contact region (114). The first inlet (106) of the absorptionreactor (102) is a flue gas inlet through which a CO₂ rich flue gasenters the absorption column (102). The second inlet (108) is anabsorbent solution inlet for the aqueous absorbent (such as thesubstituted heteroaromatic absorbent solutions hereinbefore described)through which a CO₂ lean absorbent enters the absorption column (102).The CO₂ rich flue gas and the CO₂ lean absorbent contact in the gasabsorption contact region (114). In this region the CO₂ in the CO₂ richflue gas is absorbed into the absorbent solution where it is bound insolution to form a CO₂ lean flue gas and a CO₂ rich absorbent solution.

In conducting the process of the invention the aqueous composition maycomprise a mixture of optionally substituted heteroaromatic compounds.

The local environment of the solution may be altered in the absorptioncolumn to favour the absorption reaction, e.g. to increase absorption ofCO₂ into solution where it is bound to the substituted heteroaromaticcompound. Such alterations of the local environment may include a changein pH, a change in solution temperature, a change in pressure etc.Alternatively, or additionally, the solution may include other compoundswhich assist in the absorption of CO₂. These compounds may alter theaffinity or absorption capacity of the substituted heteroaromaticcompound for CO₂, or these compounds may also absorb CO₂.

If additional compounds are added to the absorbent solution in theabsorption reactor (102), the process may additionally include means toremove these compounds.

The absorption of CO₂ from the CO₂ rich flue gas into the absorbentsolution results in a CO₂ lean gas and a CO₂ rich absorbent solution.The CO; lean gas may still include some CO₂, but at a lowerconcentration than the CO₂ rich flue gas, for example a residualconcentration of CO₂.

The CO₂ lean gas leaves the absorption column (102) through the firstoutlet (110), which is a CO₂ lean gas outlet. The CO₂ rich absorbentsolution leaves the absorption column through the second outlet (112),which is a CO₂ rich absorbent outlet.

The aqueous composition may, if desired, include solvents in addition towater in order to modify solubility of the substituted aromatic compoundand/or other absorbents which may be present composition. Examples ofcosolvents may, for example, be selected from the group consisting ofglycols, glycol derivatives selected from the group consisting of glycolethers, glycol ether esters, glycol esters, long chain short chainaliphatic alcohols such as C₁ to C₄ alkanols, long chain aliphaticalcohols, long chain aromatic alcohols, amides, esters, ketones,phosphates, organic carbonates and organo sulfur compounds.

Desorption reactor (104) includes an inlet (118), a first outlet (120),a second outlet (122), and a gas desorption region (124). The CO₂ richabsorbent outlet (112) of the absorption column (102) forms the inlet(118) of the desorption column (104). Desorption of CO₂ from the CO₂rich solution occurs in the gas desorption region (124).

Desorption of CO₂ from the CO₂ rich solution may involve the applicationof heat or a reduction in pressure to favour the desorption process.Furthermore, additional compounds may be added to the CO₂ rich solutionto enhance the desorption process. Such compounds may alter the solutionenvironment, for example by changing solution pH or altering anotherparameter to favour the desorption reaction.

Removal of CO₂ from the CO₂ rich solution results in the formation of aCO₂ lean gas stream and a CO₂ lean absorbent solution. The CO₂ leanabsorbent solution may still include some CO₂, but at a lowerconcentration than the CO₂ rich solution, for example a residualconcentration of CO₂.

The CO₂ gas stream is taken off via the first outlet (120), which is aCO₂ outlet. The CO lean absorbent solution is taken off via the secondoutlet (122), which is a CO₂ lean absorbent solution outlet. The CO₂lean absorbent is then recycled and fed through the second inlet (108)to the absorption column (102).

The invention may be used to absorb carbon dioxide from gas streamshaving a wide range of carbon dioxide concentration such as from 1volume % to 99 volume % carbon dioxide. The invention is of particularlypractical use in the absorption of carbon dioxide from gas streams, suchas flue gas stream, resulting from combustion for fossil fuels such ascoal, oil and gas. Typically the carbon dioxide content of such gasstreams is in the range of from 3 volume % to 30 volume %. The inventionis particularly suited to the capture of carbon dioxide from combustionof fossil fuels and having a carbon dioxide content in the range of from8 volume % to 20 volume %. Levels of carbon dioxide in the range from 8volume % to 20 volume % are typically present in the flue gas streamfrom combustion of fuel gas, fuel oil and coal.

The invention may also be used in capture of other acid gases togetherwith carbon dioxide, such as sulfur dioxide, which may be present in gasstreams from combustion of fossil fuels from specific geologicalsources. The process of the invention provides removal of a substantialamount of CO₂ from the gas stream. For example, in some embodiments,greater than or equal to 50% by volume (vol %); specifically greaterthan or equal to 70 vol %. Following removal of carbon dioxide from thegas stream in accordance with the invention the lean carbon dioxide gasstream from fossil fuel combustion typically contains no more than about2 volume % carbon dioxide and more preferably no more than 1.5 volume %carbon dioxide.

The invention will now be described with reference to the followingExamples. It is to be understood that the examples are provided by wayof illustration of the invention and that they are in no way limiting tothe scope of the invention.

EXAMPLES

The chemical abbreviations used in the specification have the followingmeaning:

-   AMPy: (aminomethyl)pyridine-   2-AMPy: 2-(aminomethyl)pyridine-   3-AMPy: 3-(aminomethyl)pyridine-   4-AMPy: 4-(aminomethyl)pyridine-   MEA: monoethanolamine-   AMP: 2-amino-2-methyl-1-propamol-   BZA: benzylamine-   DMEA: dimethylethanolamine

Example 1

2-AMPy, 3-AMPy, and 4-AMPy were evaluated for CO₂ mass transfer ratestogether with blends of the amines with monoethanolamine (MEA),2-amino-2-methyl-1-propanol (AMP), and N,N-dimethylethanolamine (DMEA).

AMPy's have larger CO₂ absorption rates than MEA at low CO₂ loadings.Above 0.3 CO₂ loading, CO₂ absorption rates fall below MEA. AMPy'sabsorption rates are faster than sterically hindered and tertiary amineabsorbents. Equimolar blending with MEA results in faster CO₂ absorptionrates at low CO₂ loadings at a similar overall concentration to MEAwhile maintaining a similar absorption rate to MEA at high CO₂ loadingsabove 0.3.

AMPy's and their blends were evaluated here for their CO₂ absorptionrates using a wetted-wall column contactor.

Experimental Method

A general schematic of the Wetted Wall Column (WWC) apparatus is shownin FIG. 2a and an expanded view of the column portion of the WWC isshown in FIG. 2b . The WWC apparatus (1) is comprised of a first portion(10) comprising a hollow stainless steel column (11) extending from abase (12) having an inlet (13) for gas stream containing carbon dioxideand received in a second housing portion (20) comprising a temperaturecontrolled jacket (21) and a gas outlet (22). The column (11) has aneffective height and diameter of 8.21 cm and 1.27 cm respectively. About0.65 L of amine solution within in a submerged reservoir held in atemperature controlled water bath is pumped up the inside (14) of thecolumn (11) before exiting through small outlet holes (15) in the top(16) of the column (11). The exiting liquid falls under gravity over theoutside (17) of the column (11) forming a thin liquid film beforecollecting at the base (12) of the column portion (10). The solution isreturned from an outlet (19) on the base (12) to a reservoir in a closedloop configuration. Thus, the liquid flowing over the outside (16) ofcolumn (12) is constantly replenished with fresh amine solution from thereservoir. The temperature of the column (20) and surrounding gas space(23) is controlled by a glass jacket (21) connected to the water bath.

The total liquid flow rate within the apparatus was maintained at 121.4mL·min⁻¹ (2.02 mL·s⁻¹) as indicated by a calibrated liquid flow meter. Amixed CO₂/N₂ gas was prepared by variation of Bronkhorst mass flowcontrollers for CO₂ and N₂ respectively to achieve a total gas flow rateof 5.0 L min⁻¹. Prior to entering the column (12) the gas stream ispassed through a ⅛″ steel coil and saturator located in a water bath.Liquid and gas flow rates were selected to achieve a constant smooth andripple free liquid film on the outside of the column (12). Theabsorption flux into the amine solutions, N_(CO2), was measured as afunction of dissolved CO₂ loadings from 0.0-0.4 moles CO₂/total moleamine and over a range of CO₂ partial pressures spanning 1.0-20.0 kPa.The composition of the gas stream prior to and exiting the housing (20)was monitored via a Horiba VA-3000 IR gas analyser. CO₂ loaded aminesolutions were prepared by bubbling a pure CO₂ gas stream into a knownvolume of amine solution and the resulting mass change in the solutionused to indicate CO₂ loading. A tower of condensers was connected to theoutlet of the flask to ensure loss of amine and water vapour wasminimised.

The amount of CO₂ absorbing into the amine liquid was determined fromthe CO₂ content of the gas stream entering (bottom) and exiting (top)the housing (20). The former was measured while bypassing the absorptioncolumn with the gas stream passing directly to the gas analyser. Theabsorption flux, expressed in millimoles (mmoles) of CO₂ absorbed persecond per unit area of contact between liquid amine and gas, wasdetermined over a range of CO₂ partial pressures in each of the aminesolutions and CO₂ loadings.

CO₂ mass transfer co-efficients, K_(G), incorporate the processes ofphysical absorption and chemical reaction, into a single value. Ideally,absorbents with larger CO₂ mass transfer rates result in smallerabsorption equipment and significant cost reductions.

K_(G) values as a function of CO₂ loading are presented in 1, FIG. 3 andFIG. 4. From the curves in the figures CO₂ mass transfer is faster in4-AMPy solutions at low CO₂ loadings compared to MEA at similarconcentrations. The absorption rate declines with increasing CO₂ loadingand is 50% lower than MEA towards higher loadings of 0.4. In practicethis does not present an issue due to the faster absorption at low CO2loading which offsets the lower rate at high loading. The larger cycliccapacity also means the AMPy's tend to more easily be stripped to lowloadings where the mass transfer is shown in Table 4.

The trend in K_(G) at low CO₂ loadings follows 2-AMP>3-AMPy>4-AMPy. Thesuperior reactivity of 2 and 3-AMPy over 4-AMPY respectively is believedto extend from the increased basicity of the amines. CO₂ mass transferis similar among the AMPy's at high CO₂ loadings. Equimolar blends of3.0M 3-AMPY with DMEA and AMP result in lower CO₂ mass transfer ratesthan 6.0M MEA over the entire CO₂ loading range. A similar blend of4-AMPy with MEA results in increased CO₂ mass transfer at low CO₂loadings (˜35% at 0.0 CO₂ loading) and similar CO₂ mass transfer at highloadings.

TABLE 1 Overall CO₂ mass transfer co-efficients, K_(G) for(aminomethyl)pyridines and their blends. Overall CO₂ mass CO₂ loadingtransfer co-efficient Absorbent composition (moles CO₂/ K_(G) (mmol ·m⁻² · (M) total mole amine) ^(s−1) · Pa⁻¹) 4-(aminomethyl)pyridine 3.0M4-AMPγ 0 1.9 0.2 1.49 0.4 0.677 5.0M 4-AMPγ 0 2.706 0.2 1.545 0.373 0.686.0M 4-AMPγ 0 2.86 0.2 1.75 0.4 0.412 3.0M 4-AMPγ + 3.0M MEA 0 3.25 0.21.994 0.4 0.83 3.0M 4-AMPγ + 3.0M AMP 0 2.197 0.201 1.392 0.406 0.5723.0M 4-AMPγ + 3.0M DMEA 0 2.364 0.2 1.444 0.4 0.57993-(aminomethyl)pyridine 3.0M 3-AMPγ 0 1.77 0.2 1.462 0.4 0.465 5.0M3-AMPγ 0 2.74 0.2 1.57 0.4 6.0M 3-AMPγ 0 3.19 0.2 1.77 0.4 0.35 3.0M3-AMPγ + 3.0M MEA 0 3.56 0.2 2.16 0.4 0.702 3.0M 3-AMPγ + 3.0M AMP 02.71 0.2 1.51 0.4 0.514 2-(aminomethyl)pyridine 3.0M 2-AMPγ 0 2.26 0.21.55 0.4 0.593 5.0M 2-AMPγ 0 2.88 0.2 1.82 0.4 0.459 6.0M 2-AMPγ 0 3.570.2 1.85 0.4 0.326

Importantly, blending with DMEA and AMP results in similar viscositiesto standalone AMPy absorbents at similar concentrations and low (orzero) CO₂ loading. Viscosity of the 4-AMPy/AMP blend increases with CO₂loading similarly to the standalone 6.0M 4-AMPy absorbent while theviscosity of the blend with DMEA increases at a slower rate (˜40% lowerviscosity at 0.4 loading). Viscosity of the 4-AMPy/MEA blend issubstantially lower at all CO₂ loadings (˜75% lower viscosity at 0.4 CO₂loading). A number of the AMPy/MEA blends exhibited similar or largerCO₂ mass transfer rates than the standalone absorbents indicating thatblended absorbents are useful.

The larger physical CO₂ solubility in the AMPy's makes up for the lowerreaction kinetics.

TABLE 2 Protonation constants from 15.0-45.0° C. together withcorresponding enthalpies and entropies for 2-(aminomethyl)pyridine,3-(aminomethyl)pyridine, and 4-(aminomethyl)pyridine respectively. BZAdata included for comparison. Temperature (° C.) ΔH° ΔS° 15 25 35 45 kJ· mol⁻¹ J · mol⁻¹ · K⁻¹ 1/T (K) 0.00347 0.00335 0.00324 0.00314 4-AMPylog K prot 8.77 8.43 8.08 7.86 −53.9 19.6 log K carb Prot 2 3.84 3.653.48 3.41 −25.5 15.3 3-AMPy log K prot 9.11 8.83 8.51 8.26 −50.6 0.8 logK carb Prot 2 3.42 3.34 3.25 3.17 −14.5 −15.2 2-AMPY log K prot 8.988.68 8.41 8.19 −46.6 −10.1 log K carb Prot 2 1.96 1.86 1.75 −17.9 24.5BZA* log K prot 9.77 9.43 8.95 8.61 −59.5 20 *Single protonation only

The table shows the larger enthalpy of reaction for the AMPy's whichleads to more CO₂ release with increasing temperature. This contributesto the good cyclic capacity of the substituted heteroaromatic compounds.

From the data in Table 2 the first protonation constants of AMPy, log Kprot are similar to MEA and BZA and fall within a suitable range for CO₂capture processes. Importantly, the large and desired protonationenthalpy (ΔH⁰) observed for BZA is largely maintained in the AMPyderivatives (−53.9 to −46.6). This large enthalpy is a unique featureand is responsible for the superior cyclic capacity compared to othernon-aromatic and cyclic amine absorbents.

TABLE 3 4-AMPy carbamate stability constants from 15.0-45.0° C. togetherwith corresponding enthalpies and entropies for 4 and3-(aminomethyl)pyridine. Temperature (° C.) ΔH° ΔS° 15 25 35 45 kJ ·mol⁻¹ J · mol⁻¹ · K⁻¹ 1/T (K) 0.00347 0.00335 0.00324 0.00314 4-AMPy logK carb 2.35 2.22 2.02 1.84 −30.05 −58.97 log K carb Prot 7.15 6.95 6.696.52 −38.08 4.91 3-AMPy log K carb 2.34 2.11 1.93 1.70 −36.83 82.97 BZAlog K carb 2.20 2.01 1.81 1.58 −36 −82 MEA log K carb 1.85 1.76 1.661.55 −18 −35

TABLE 4 CO₂ loadings and cyclic capacities for 6.0M solutions of 4-AMPy,MEA, and BZA respectively together with data for an equimolar blendcontaining 3.0M 4-AMPy and 3.0M MEA. CO₂ loading @ 15 kPa CO₂ cyclicTemperature (° C.) 40 120 capacity 6.0M 4-AMPy 0.50 0.10 0.40 6.0M MEA0.52 0.32 0.20 6.0M BZA 0.50 0.13 0.37 3.0M 4-AMPy + 3.0M MEA 0.50 0.230.27

The larger cyclic capacity of the (aminomethyl)pyridines produces loweroperating energy requirements of the process. As a result less absorbentis required to capture the same amount of CO₂ thereby loweringcirculation rates and reboiler duties.

Example 2: Benzylamine Comparison

Benzylamine (BZA) has been found to form solid precipitate salts in thepresence of CO₂, particularly at high concentrations of BZA in theliquid phase, limiting the optimum operating concentration to <30 wt %(˜2.8 M). It was also found that BZA has the propensity to form smallamounts of solid precipitates in the gas phase due to non-ideal liquidbehaviour leading to elevated vapour pressures above those predicted byRaoult's law at low CO₂ loadings. As part of the absorbent screeningprocess concentrated aqueous 3-AMPy was evaluated for its potential toform solid precipitates in the presence of CO₂ at high concentrations.While similar in structure to BZA the incorporation of a second nitrogengroup into the aromatic ring was found to increase solubility of thecarbamate product in aqueous solution. A simple bubble reactor wasutilised to determine the precipitation propensity while passing a pureCO₂ gas stream at atmospheric pressure into a concentrated 3-AMPysolution held at ambient temperature. A concentration of 80 wt % 3-AMPywas achieved before precipitation of the carbamate salt was observed.While a precipitate was observed, it is unlikely this will occur in thepilot plant given the aggressive conditions employed in the simple labbased study. Furthermore, similar precipitation behaviour has beenobserved for other common aqueous amines in highly concentratedsolutions but which are operated successfully in pilot plants.

Example 3: VLE Measurements for AMPy Derivatives and Blends

The capacity of an absorbent solution for CO₂ is a vital requirement forabsorbent development. CO₂ capacity drives the optimum absorbentconcentration, energy requirements, and liquid equilibrium (VLE)measurements are typically used to determine absorption capacities,cyclic capacities (temperature dependence), and absorption enthalpies(temperature dependence of CO₂ solubility at a given CO₂ loading).Vapour liquid equilibrium also can be predicted from knowledge of theequilibrium constants for CO₂/H₂O chemistry, amine protonation,carbamate stability, and physical CO₂ solubility (Henry's constant). VLEdata can also be used to regress equilibrium constants for carbamatestability which can be used to verify those determined from independentNMR studies (rarely available for bespoke absorbents).

The vapour liquid equilibrium apparatus used here incorporates a sealedstainless steel reactor (4 independent vessels) and high pressure CO₂gas delivery system. Absorbent liquid (5.0 mls) is placed into vesselsand evacuated several times to remove any residual CO₂ gas that may bepresent. The vessels are charged with high pressure nitrogen (˜2.0 BARabsolute pressure) before the mass is recorded using a milligrambalance. Once weighed the vessel pressure sensor is attached and theentire vessel placed into an oven at the desired temperature (40, 60,80° C.). Following overnight equilibration, the initial pressure of thevessel (now incorporating contributions from N₂ and H₂O) is recorded andthe vessel charged with CO₂ (˜5.0 BAR absolute pressure). The vessel isweighed and the mass of CO₂ dosed into the vessel recorded. The vesselis returned the oven and equilibrated for 24 hours or until steady stateis reached (indicated by no further changes in pressure). Samples fromthe vessel headspace are then analysed by gas chromatography todetermine the gas phase CO₂ partial pressure. The liquid phase CO₂loading is then determined by mass balance calculation.

The results for 6.0M 3-AMPy are shown in Table 5 below:

22° C. 40° C. 60° C. CO₂ CO₂ CO₂ mol Partial mol Partial mol PartialCO₂/mol Pressure CO₂/mol Pressure CO₂/mol Pressure 3-AMPγ (KPa) 3-AMPγ(KPa) 3-AMPγ (KPa) 0.43 0.53 0.43 2.90 0.43 16.53 0.41 0.34 0.39 1.140.38 5.70 0.39 0.21 0.34 0.51 0.35 3.29 0.37 0.12 0.31 0.29 0.27 1.400.35 0.097 0.17 0.064 0.21 0.67 0.32 0.082 0.12 0.24 0.28 0.043 0.110.19 0.26 0.093 0.022 0.030 0.22 0.017

Example 4: Energy Performance and Mass Transfer

A concentrated 3-AMPy absorbent operating at 6.0M was selected as thebase case given its simplicity. Alternative blends incorporatingequimolar concentrations of monoethanolamine (MEA) and2-amino-2-methyl-1-propanol (AMP) with 3-AMPy (i.e. 3.0M 3-AMPY+3.0M MEAor 3.0M AMP respectively) were selected to provide rapid kinetic orimproved thermodynamics while minimising the cost of the absorbentinventory for large scale evaluation in a pilot plant.

FIG. 7 of the drawings shows the model estimated reboiler duties as afunction of the ratio of liquid and gas flow rates (UG) for an isobaricstripping column with a range of aqueous absorber solutions including 6M3-AMPy and various ratios of 3-AMPy and AMP (with 30 wt % MEA includedfor comparison). These represent some of the more practically usefulformulations in terms of energy requirement, with formulationsincorporating MEA having poorer energy performance. There is littleimpact from the addition of AMP. AMP was, however, found to have asignificant positive effect on mass transfer.

Operation of the aqueous 6M 3-AMPy absorbent was completed over about 1month in a pilot plant of general operation shown in FIG. 1. Aparametric study was undertaken to estimate the CO₂ stripping energyrequirement as a function of operating parameters for a traditionalabsorber-stripper process design. Overall favourable results were foundfor the energy requirement as shown in the figure below.

Unlike the previously tested formulation based on BZA, no operationalissues were encountered with an aqueous solution of 6M 3-AMPy. Noprecipitation or foaming events occurred nor were there any issuesregarding volatility.

As can be seen in FIG. 8 the energy requirement using aqueous solutionsof (aminomethyl)pyridines such as 6M 3-AMPy is much lower than formonoethanolamine (MEA).

Example 5: Pilot Plant Trials Using 3-(Aminomethyl)Pyridine (3-AMPy)Based Absorbents

Extended pilot plant trials have been undertaken using a 0.4 tonne/day—CO₂ capture plant located at a brown coal power station. The captureplant was operated with a flue gas slip-stream of flow rate 80 m³/hrdirectly taken from the power station.

Example 5a: Pilot Plant Trials Using Absorbent Containing 6 Mol/LAqueous 3-AMPy (61 wt % 3-AMPy and 39 wt % Water)

The campaign with 6 mol/L Aqueous 3-AMPy was operated for a duration ofapproximately 1500 hours. During operation the performance of the plantwas assessed in terms of reboller energy requirement and the degradationof the amine was monitored. The single dominant degradation productformed was also identified and characterised. Minimum reboiler duties of2.9 and 2.6 GJ/tonne CO₂ without and with use of the rich split processconfiguration respectively were achieved. This is compared to 3.3GJ/tonne CO₂ for 5 mol/L monoethanolamine (MEA) in both configurations.

In laboratory testing under accelerated degradation conditions thedominant degradation product formed was found to be an imine dimer of3-AMPy. Monitoring of loss of amine and formation of the imine wasundertaken during the campaign by infrared (IR) spectroscopy and highperformance liquid chromatography (HPLC). Additional analysis of plantsamples by ¹³C and ¹H-NMR spectroscopy confirmed that the previouslyidentified imine was the primary degradation product in the plant.

FIG. 9 is a plot of the trend in amine and imine concentration duringthe pilot plant campaign.

During the pilot plant trial the degradation reaction mechanism wasinvestigated in the laboratory by breaking the overall reaction downinto the possible individual chemical transformations and testing ifthey occurred. The determined mechanism proceeds via a protonated 3-AMPymolecule and loss of an ammonium ion followed by oxidation. The completeof degradation mechanism of 3-AMPy to an imine via reaction with oxygenis shown in the following Scheme.

Example 5b: 3 Mol/L 3-AMPy and 3 Mol/L 2-Amino-2-Methyl-1-Proponal (AMP)(32 wt % 3-AMPy, 26 wt % AMP and 42 wt % Water)

Based on the degradation mechanism of the above scheme it was consideredthat if 3-AMPy was formulated with a stronger base to reduce theformation of protonated 3-AMPy during CO₂ absorption, its degradationcould be suppressed. AMP was chosen as the amine for formulation as ithas the required basicity and is known to be robust in CO₂ captureapplications. AMP does not react directly with CO₂ but rather acts as abase to preferentially accept the protons released when 3-AMPy reacts.Simulations indicated that the concentrations used in the absorbent ofaqueous 3 mol/L 3-AMPy and 3 mol/L 2-amino-2-methyl-1-proponal (AMP)were optimal to reduce degradation and maintain capture performance.

A pilot plant campaign was conducted with the aqueous absorbent ofExample 5b for 5000 hours. It was possible to conduct a much longercampaign as the degradation of the absorbent was much slower than theabsorbent of Example 5a. The same reboiler energy requirements wereachieved as in Example 5a. FIG. 10 shows the concentrations of 3-AMPy,AMP and imine over the duration of the campaign. The rate of imineformation was orders of magnitude lower than seen in the trial ofExample 5a. In addition the overall rate of degradation of was found tobe 10 times slower than 5 mol/L MEA.

The formation of the imine dimer and inhibition of degradation is aunique property of aminomethylaromatic systems and in particularenhances the performance on aminomethyl substituted heteroaromatics.Aminoalkylpyridine with longer bridging chains such as ethyl and propylbetween the amino and pyridine group do not form the imine and degradevia chain loss and more traditional mechanism that form products thatcannot be easily regenerated.

FIG. 10 3-AMPy, AMP and imine concentrations measured during the pilotplant campaign by IR spectroscopy over 5000 hours of operation.

In contrast with the reduction in the concentration of 3-AMPy shown inFIG. 9 the composition of Example 5b consisting of aqueous 3 mol/L3-AMPy and 3 mol/L 2-amino-2-methyl-1-proponal (AMP) (32 wt % 3-AMPy, 26wt % AMP and 42 wt % water) showed little reduction in the concentrationof the amines over the period of 5000 hours operation of the pilot plantunder the same conditions.

Example 6: Comparison of the Absorbent of Example 5 with MEA

The pilot plant was also operated with 5 mol/L (30 wt %) aqueousmonoethanolamine (MEA) for approximately 500 hours. This allowed optimumreboiler duties to be identified for each absorbent via parametricstudy. These optimum reboiler duties and rates of amine degradation areshown in Table 6 below and are for the standard plant configuration (norich split). Note that the MEA degradation information is taken fromliterature as it was only run for a short duration in the pilot plant.

TABLE 6 5 mol/L MEA Example 5a Example 5b Reboiler duty 3.4 2.9 2.9(GJ/tonne CO₂) Amine degradation 1.5* 23 0.16 rate (kg/tonne CO₂)*Degradation rate taken from book P. Feron, Absorption-BasedPost-Combustion Capture of Carbon Dioxide, Elsevier (2016).

Example 7: Anilne/Aminopyridines

Aniline and aminopyridine compounds lack the basicity required toeffectively act as CO₂ absorbents. The pKa of their conjugate acids issmaller than or similar to that of CO₂ in aqueous solution (6.3 at 25°C., T J Edwards, G Maurer, J Newman, J M Prausnitz; AIChE J., 24,966(1978)). The pKa of the conjugate bases is shown in Table 7 below. Thusthey are unable to accept protons from CO₂ ionisation. In addition theywill only directly react with CO₂ to form a carbamate in the presence ofa strong base (P V Kortunov, L S Baugh, M Siskin, D C Calabro; Energy &Fuels, 29,5967 (2017)). This lower basicisity compared to aliphaticamines is due to the delocalisation of the lone pair of electrons on thenitrogen into the aromatic ring. 4-Aminopyridine is a special case dueto the larger pK_(a) of its conjugate acid, which is in a suitable rangefor CO₂ absorption. This is due to resonance structure stabilisation ofthe ion, but this stabilisation renders it unable to react directly withCO₂ to form a carbamate (A Albert, R Goldacre, J Phillips; J. Chem.Soc., 455,2240 (1948)).

TABLE 7 pK_(a) of conjugate acid at Compound 25° C. in water Aniline 4.62-Aminopyridine 6.7 3-Aminopyridine 6.3 4-Aminopyridine 9.52-(Aminomethyl)pyridine 8.7 3-(Aminomethyl)pyridine 8.84-(Aminomethyl)pyridine 8.4

Example 8: Stability of aminoalkylaromatics

Relationship between structural rigidity and CO₂ capture performance.

The enthalpy of protonation is an important parameter in aqueous amineabsorbents. The larger the enthalpy of protonation the larger the CO₂cyclic capacity that can be achieved via a temperature swing process. Arelationship exists between the enthalpy and entropy of protonation andit is described in the publication D Fernandes, W Conway, X Wang, RBurns, G Lawrance, M Maeder, G Puxty; J. Chem. Thermodynamics, 51,97(2012). In summary, the more structurally rigid a molecule, the morenegative the enthalpy of protonation and smaller and/or more negativethe entropy of protonation. This effect is due to reduced internaldegrees of freedom resulting in smaller entropy changes uponprotonation. Thus aminomethyl substituted heteroaromatic groups havemore favourable enthalpy of protonation properties than longer alkychain bridging groups such as ethyl or propyl bridging groups oraliphatic amines. FIG. 11 compares the enthalpy and entropy ofprotonation of a range of amines. The red squares are allow comparisonof 4 different aminomethyl aromatic compounds to 2-phenyethylamine toillustrate this relationship.

Example 9: Bases to Prevent Degradation of (Aminomethyl)Pyridines

As shown in Example 5b the presence of AMP in the absorber compositioninhibited the formation of the imine produced by dimerization of theAMPy derivative formed on absorption of CO₂. Other bases includingamines of higher pKa than the (aminomethyl)pyridine may also be used inthis role. Preferred bases are tertiary and sterically hindered amineswhich are stable and provide a proton accepting role on CO₂ absorption.The average pKa of 2-(aminomethyl)pyridine, 3-(aminomethyl)pyridine and4-(aminomethyl)pyridine at 25° C. is 8.6

Suitable bases typically have a pKa at least 0.25 units higher than thepKa of the (aminomethyl)pyridines, that is about 0.25 units higher than8.6 (this represents a 2.5× increased selectivity for protons). Examplesof suitable tertiary and sterically hindered amines include thosespecified in Table 8 together with the pKa at 25° C.

TABLE 8 Base pK_(a) at 25° C. 2-amino-1-propanol 9.52-amino-2-methyl-1-propanol 9.7 piperidine 11.1 2-piperidinylmethanol10.1 3-pipendinylmethanol 10.4 4-pipendinylmethanol 10.62-piperidinylethanol 10.5 4-piperidinyiethanol 10.62-(dimethylamino)ethanol 9.2

1. A process for absorbing carbon dioxide from a gas stream containingcarbon dioxide, comprising contacting the gas stream with an absorbentcomprising an aqueous composition comprising at least 10 wt % water anda substituted heteroaromatic compound selected from the group consistingof formula Ia, Ib, Ic, Id and mixtures of two of more thereof:

wherein R¹ is methylene; R⁴ is an optional carbon substituent selectedfrom the group consisting of C₁ to C₄ alkyl, hydroxy, hydroxy-C₁ to C₄alkyl, C₁ to C₄ alkoxy, C₁ to C₄ alkoxy-(C₁ to C₄ alkyl; and n is 0, 1or
 2. 2. (canceled)
 3. (canceled)
 4. The process of claim 1, wherein thesubstituted heteroaromatic compound is of formula Ia:

wherein R¹ is methylene; R⁴ is an optional carbon substituent selectedfrom the group consisting of C₁ to C₄ alkyl, hydroxy, hydroxy-C₁ to C₄alkyl, —C₁ to C₄ alkoxy, and C₁ to C₄ alkoxy-(C₁ to C₄ alkyl); and n is0, 1 or
 2. 5. (canceled)
 6. (canceled)
 7. (canceled)
 8. The process ofclaim 1, wherein the substituted heteroaromatic compound is selectedfrom the group consisting of: 2-(aminomethyl)pyridine,3-(aminomethyl)pyridine and 4-(aminomethyl)pyridine.
 9. The process ofclaim 1, wherein the substituted heteroaromatic compound is3-(aminomethyl)pyridine.
 10. The process of claim 1, wherein theconcentration of substituted heteroaromatic compound is 1 wt % to 80 wt% of the aqueous solution.
 11. The process of claim 1, wherein thesubstituted heteroaromatic compound is dissolved in the aqueous solutionin an amount of at least 20 wt % of the water content of the aqueoussolution.
 12. The process of claim 1, wherein the concentration of thesubstituted heteroaromatic compound is 10 wt % to 80 wt % of the aqueoussolution.
 13. The process of claim 1, wherein the aqueous compositioncomprises an additional amine selected from tertiary amines, hinderedamine and mixture thereof having a pKa greater than that of thesubstituted heteroaromatic compound.
 14. The process of claim 1, whereinthe composition further comprises a tertiary amine, hindered amine ormixture thereof having a pKa at least 8.85 at 25° C.
 15. The processaccording to claim 1, wherein the aqueous composition comprises a carbondioxide absorbent which is selected from the group consisting ofImidazole and N-functionalised imidazole.
 16. The process of claim 1,comprising the substituted heteroaromatic compound in an amount of 10 wt% to 80 wt % of the aqueous solution and further absorbent wherein theweight ratio of the substituted heteroaromatic compound to furtherabsorbent is from 1:10 to 10:1.
 17. The process of process of claim 1,wherein the water content of the aqueous solution is at least 15 wt %.18. The process of claim 1, wherein the absorbent comprises 10 wt % to60 wt % of (aminomethyl)pyridine comprising one or more of2-(aminomethyl)pyridine, 3-(aminomethyl)pyridine and4-(aminomethyl)pyridine; 10 wt % to 60 wt % of amine selected fromtertiary amines and sterically hindered primary and secondary amines andmixtures thereof of pKa at least 8.85; and 20 wt % to 80 wt % water. 19.The process of claim 18, wherein the amine selected from tertiary aminesand primary and secondary sterically hindered amines is selected fromthe group consisting of 2-amino-1-propanol, 2-amino-2-methyl-1-propanol,piperidine, 2-piperidinylmethanol, 3-piperidinylmethanol,4-piperidinylmethanol, 2-piperidinylethanol, 4-piperidinylethanol,2-(dimethylamino)ethanol and mixtures of two or more thereof.
 20. Acomposition of adsorbed carbon dioxide comprising: A. an aqueous solventcontaining water in an amount of 10 wt % of the composition; B. at leastone absorbent compound for carbon dioxide comprising a substitutedheteroaromatic compound selected from the group consisting of formulaIa, Ib, Ic, Id and mixtures of two or more thereof;

wherein R¹ is methylene; R⁴ is an optional carbon substituent selectedfrom the group consisting of C₁ to C₄ alkyl, hydroxy, hydroxy-C₁ to C₄alkyl, C₁ to C₄ alkoxy, C₁ to C₄ alkoxy-(C₁ to C₄ alkyl); and n is 0, 1or 2; and C. absorbed carbon dioxide, wherein the absorbed carbondioxide is at a concentration above the equilibrium concentration whenthe solution is exposed to air at below the boiling point of thesolvent.
 21. The composition of claim 20, wherein the absorbed carbondioxide constitutes at least 1% absorbed carbon dioxide by weight basedon the total weight of the solution.
 22. The composition of claim 20,wherein the substituted heteroaromatic compound is selected from thegroup consisting of: 2-(aminomethyl)pyridine; 3-(aminomethyl)pyridine;4-(aminomethyl)pyridine.
 23. The composition according to claim 20,wherein, the solution comprises one or more amine absorbents, inaddition to the substituted heteroaromatic compound.
 24. The compositionof claim 20, wherein the concentration of substituted heteroaromaticcompound is 1 wt % to 80 wt % of the aqueous solution.
 25. Thecomposition of claim 20, wherein the absorbent solution comprises 10 wt% to 60 wt % of (aminomethyl)pyridine comprising one or more of2-(aminomethyl)pyridine, 3-(aminomethyl)pyridine and4-(aminomethyl)pyridine; 10 wt % to 60 wt % of amine selected fromtertiary amines and primary and secondary sterically hindered amines andmixtures thereof of pKa at least 8.85; and 20 wt % to 80 wt % water. 26.(canceled)
 27. (canceled)